Oscillation isolation systems such as these with pneumatic isolators are known from the prior art. For example, EP 0 927 380 B1=US 6226 075 B1 describes an air bearing which acts horizontally and vertically and which is used, for example, to support a lithographic system as the mass. Furthermore, actuators which can be driven electrically and support the mass parallel to the pneumatic isolators are used to suppress oscillations that occur.
Pneumatic bearings, also referred to as isolators, are particularly suitable for designing an oscillation isolation system because they allow the bearing stiffness to be kept very low. However, they result in a continuous leakage flow from the air bearing, which must be made good. Air bearings such as these also have the advantage that the total oscillation-isolated mass can be moved vertically, thus making it possible to move to one or more operating points. In order to allow this, the isolators have valves which can be driven electrically and use an electrical control signal to control the compressed-air flow from a compressed-air supply to the isolators, in this way allowing the air pressure in the isolators to be increased or decreased, or to be maintained at the desired level.
The compressed-air flow control for the valves according to the prior art is subject to hysteresis. This means that the same electrical drive signal does not always produce the same pressure in the pneumatic isolator. In fact, the pressure that is produced depends on the drive signal that was previously applied to the valve. The reason for this valve behavior is that the valves contain mechanical elements in order to control the compressed-air flow and these are subject to mechanical friction. The hysteresis effect results in non-linearity and this cannot be detected by simple control systems. Undesirable effects occur, therefore, during stabilization of the oscillation isolation system, since the moving mechanical element of the relevant valve must be moved repeatedly from different initial states, in particular away from the steady state.